Fabrication of microstructured optical fibre

ABSTRACT

Microstructured optical fibre is fabricated using extrusion. The main design of optical fibre has a core suspended in an outer wall by a plurality of struts. A specially designed extruder die is used which comprises a central feed channel, flow diversion channels arranged to divert material radially outwards into a welding chamber formed within the die, a core forming conduit arranged to receive material by direct onward passage from the central feed channel, and a nozzle having an outer part in flow communication with the welding chamber and an inner part in flow communication with the core forming conduit, to respectively define an outer wall and core of the preform. With this design a relatively thick outer wall can be combined with thin struts (to ensure extinction of the optical mode field) and a core of any desired diameter or other thickness dimension in the case of non-circular cores. As well as glass, the extrusion process is suitable for use with polymers. The microstructured optical fibre is considered to have many potential device applications, in particular for non-linear devices, lasers and amplifiers.

BACKGROUND OF THE INVENTION

The invention relates to optical fibre, more particularly to a processfor fabricating microstructured optical fibre, its preforms, tomicrostructured optical fibre made using the process and to devicesincorporating microstructured optical fibre.

Microstructured optical fibre, also frequently referred to in the art asholey fibre or photonic crystal fibre, is the subject of intensiveresearch and development.

To date, microstructured optical fibre has been manufactured by acapillary stacking process. A number of circular section rods arestacked together inside a jacket and drawn or “caned” into a preform.The preform is then drawn again into the microstructured optical fibre.

FIG. 1 of the accompanying drawings is a schematic section of aconventional microstructured fibre preform. A core rod 10 (shown assolid, but may be hollow) is surrounded by at least one ring of hollowcladding capillary tubes 12 (two rings in the figure) which in turn isenclosed in an outer jacket 14 (illustrated as thick-walled, but may bethin-walled). The initial assembly of stacked tubes and/or rod(s) fromwhich the preform is drawn will have outer dimensions of the cm scale.In the preform, i.e. after the initial drawing step, the inner diameterof the jacket may be typically of the order of 1 mm. After drawing ofthe fibre, these dimensions typically reduce by around 2-3 orders ofmagnitude.

FIG. 2 is a cross-sectional micrograph of an example microstructuredfibre made from a preform generally as shown in FIG. 1, but with fourrings of hollow cladding capillary tubes, rather than two. The largeresidual holes are formed by the hollow parts of the capillary tubes.The small residual holes are formed from the three-cornered gaps formedbetween the capillary tubes and core rod.

While successful, the capillary tube stacking process has beencriticised.

Ian Maxwell [1] points out that, because capillary tubes and rods can bestacked only in a few ways, they restrict the manufacturing process andlimit the type of structures that can be fashioned. Essentially, tubesand rods stack in a tessellating arrangement, usually hexagonally closepacked, which dictates what microstructures are achievable. As well ashexagonal close packing, square grid packing has also been demonstrated.

Another issue with the capillary tube stacking process is that there area large number of air-glass surfaces which may be problematic in thatthere is a tendency for impurity incorporation and also propagation ofsurface structural defects, such as scratches and pits, duringfabrication. It may thus be difficult to apply the capillary tubestacking process on an industrial scale, at least without full cleanroom conditions.

Another significant problem with capillary stacking is that variance inthe outer diameter of the capillaries (or rods) must be kept low, notonly from capillary to capillary, but also along the length of eachcapillary. If the variance is not controlled, the stacking faults willarise.

FIG. 3 shows the structure of a proposed microstructured optical fibrein cross-section. The structure has a circular-section core 20 ofdiameter ‘d’ suspended concentrically in a circular outer wall 22 by aplurality of thin webs or struts 24 that extend along the length of thefibre as membranes. The core diameter ‘d’ is sufficiently large tosupport optical mode guidance. The strut thicknesses and lengths aresufficiently small and long respectively to ensure that the struts donot support an optical mode. In other words the struts are dimensionedso that there is evanescent mode field decay in the struts. This designensures that the struts do not influence the coarser properties of themode guidance in the core which is thus effectively air suspended.

FIG. 4 shows in section the form of an optical fibre made according toKaiser & Astle [2] in which a rod 30 is arranged on a plate 32 embeddedin a cladding tube 34 in order to fabricate a multimode optical fibre.

The idealised structure of FIG. 3 is not compatible with usual capillarystacking approach to fabricating microstructured optical fibres. Theinventors have however realised that this kind of structure is inprinciple of a form that might be manufacturable using extrusion.

Extrusion, in the form of disc extrusion, is a known technique formanufacturing conventional optical fibre and is now briefly describedfor background.

FIG. 5 is a schematic drawing illustrating disc extrusion forfabricating conventional optical fibre. A disc 40 of core glass isarranged on top of a disc 42 of cladding glass in the upper part of anextruder die 44. The glass is then subject to downward pressure(indicated by the arrow), applied by a punch or ram which forces theglass through a circular tapered aperture formed in the lower part ofthe extruder die. As a result a rod is formed with the core glassradially inwardly disposed of the cladding glass. The rod is then usedto draw a conventional optical fibre.

FIG. 6 shows a section through the tapered rod in which the core glassis formed into a circular section core 46 and the cladding glasssurrounds it to form cladding 48.

In the general field of glass forming, extrusion has been used to makecomplicated glass structures, specifically for making thermometers.Roeder & Egel-Hess [3] describe extrusion of complicated glassstructures.

FIG. 7 is a section drawing reproduced from Roeder & Egel-Hess showingan extruder die 70 used to make a tube. The Roeder & Egel-Hess extruderdie 70 comprises a main body 72 which holds a die 74, a funnel part 76and a spider 80. A mandrel 78 is attached to the spider 80 by a fixing84. A second fixing 86 holds the main body 72, the die 74, the funnelpart 76 and the spider 80 together. A cap 82 is attached to the mainbody 72 as indicated in FIG. 7. The spider 80 defines three channels 88a, 88 b, 88 c in fluid communication with a welding chamber 90 definedby the mandrel 78 and the funnel part 76. In operation, glass is heldwithin the cap region 82 and urged through the channels 88 a, 88 b, 88 cin the spider 80 under the application of an external force in thedirection indicated by the arrow. The glass is split into three streamsby the spider 80. These streams recombine within the welding chamber 90to form a single rope, the angled walls of the funnel part 76 assistthis process by concentrating the material. The die 74 and mandrel 78together define a cylindrical section 92 through which the glass withinthe funnel part 76 is pushed The resulting extruded glass has a circularring cross-section defined by the geometry of the cylindrical section92.

FIGS. 8 a-8 d are perspective views of more complicated glass structuressuccessfully fabricated by Roeder & Egel-Hess in which a core iseffectively suspended by a plurality of struts inside an outer wall.Although these glass structures do not appear to have been made using anextruder die as shown in FIG. 7, which is designed for extruding simpletubes, perhaps the extruder dies used to make these more complexstructures were in some way modified versions of the extruder diedesigns described in the article Roeder & Egel-Hess.

Special considerations arise for microstructured optical fibrefabrication which were not relevant to the general work of Roeder &Egel-Hess that was not concerned with optical fibre fabrication, butrather thermometer glass structures.

For microstructured optical fibre fabrication the followingconsiderations need to be taken account of

optical design considerations dictate that the extrusion process shouldallow the wall thicknesses of the struts to be several times thinnerthan the core diameter so that optical mode extinction can be ensured;

fabrication considerations dictate that the extrusion process shouldallow for the outer walls to be relatively thick, meaning that the outerwall thickness is several times thicker than the thicknesses of thestruts;

the optical quality of the core glass is paramount; and

surface quality of the core glass, and of surrounding glass where themode field has significant power, is paramount.

The first two design considerations although apparently modest do infact present considerable difficulty for a glass maker familiar withextrusion. One of the major principles of extruder die design is thatall wall thicknesses should be the same. This is in order to ensure thatthe glass is forced out of the end aperture of the die uniformly acrossthe required die pattern. Surface friction in the die means that anyvariation in die aperture dimension will result in differential glassflow across the die. The general rule is to avoid any such complicationsin order to preserve integrity of the extrusion process.

The third design consideration is also not compatible with conventionaldie designs, since the glass that ultimately forms the core is notspecially treated by the die.

The fourth design consideration is considered to be novel altogether,since it is not relevant to extrusion of thermometer structures orconventional optical fibre.

It is therefore an aim of the invention to fabricate microstructuredoptical fibre and preforms by extrusion to allow novel microstructuresto be achieved that cannot be made with conventional capillary stackingmethods.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided anextruder die for forming a preform for manufacture into an opticalfibre, comprising: a central feed channel for receiving a materialsupply by pressure-induced fluid flow; flow diversion channels arrangedto divert a first component of the material radially outwards into awelding chamber formed within the die; a core forming conduit arrangedto receive a second component of the material from the central feedchannel that has continued its onward flow; and a nozzle having an outerpart in flow communication with the welding chamber and an inner part inflow communication with the core forming conduit, to respectively definean outer wall and core of the preform.

With this novel die design the multiple requirements for extrudingpreform shapes required for microstructured optical fibres can besatisfied. In particular, material feed through a central feed channelfollowed by subsequent diversion of part of the material to fill awelding chamber and continuation of another part of the material to formthe central core, allows a high optical quality core to be formed withvery smooth surfaces in the core region while at the same time allowinga thick outer wall to be made in combination with thin supportingstruts.

It is considered that the above-specified requirements cannot be metsatisfactorily with a conventional die design in which the material isforced radially inwardly from a conventional spider feed into a centralaxial region.

As detailed in the following, the use of extrusion to produce amicrostructured preform has been demonstrated. The preform has beencaned and drawn into a microstructured optical fibre which is capable ofsingle-moded light guidance over a broad range of wavelengths. Thedisclosed die design allows extrusion to be used to produce complexstructured preforms with good surface quality, and makes efficient useof raw materials. By avoiding capillary stacking, fewer interfaces areinvolved, and so ultimately extrusion may offer lower losses thanexisting techniques. In addition, extrusion can be used to producestructures that could not be created with capillary stacking approaches,and so a significantly broader range of properties should be accessiblein extruded microstructured fibres. Single-material fibre designs avoidcore/cladding interface problems, and so should potentially allowlow-loss fibres to be drawn from a wide range of glasses and polymers.

The extruder die may be provided with pairs of mutually facing internalwalls that form gaps extending between the core forming conduit and thewelding chamber and allow fluid communication therebetween, the gapsbeing shaped to form struts supporting the core in the outer wall.

The mutually facing internal walls may incorporate at least one bend inorder to increase the radial length of the struts. This is useful tocounteract the effects of surface tension when the preform is reduced bycaning and/or drawing. The mutually facing internal walls may extendparallel to each other for a part or the whole of their extent or may betapered either in the principal flow direction or in a perpendicularplane thereto.

The internal walls may have a radial length greater than the gap width.The radial length of the internal walls is greater than the gap width bya factor of one of: 2, 3, 4, 5, 6, 7, 8, 9, 10 and 20.

In some embodiments, the outer part of the nozzle is shaped to provide acircular-section preform outer wall.

In other embodiments, the outer part of the nozzle deviates from acircular shape so as to provide sections of preform wall interconnectingwall-to-strut junctions that are shorter than would be required to forma circular-section preform outer wall. This is useful to counteract theeffects of surface tension when the preform is reduced by caning and/ordrawing and may be advantageously combined with the above-mentionedbends in the internal walls.

The outer part of the nozzle preferably has a first dimension defining awall thickness of the preform outer wall and wherein said firstdimension is greater than said gap between the mutually facing internalwalls that form the preform struts. In examples, said first dimension isgreater than said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and10.

The inner part of the nozzle preferably has a second dimension defininga core thickness of the preform core and wherein said second dimensionis greater than said gap between the mutually facing internal walls thatform the preform struts. In examples, said second dimension is greaterthan said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and 10.

The flow diversion channels may include a first group of the flowdiversion channels which extend from the core forming conduit to thewelding chamber. The flow diversion channels of the first group extendperpendicular to the core forming conduit in one example. The flowdiversion channels of the first group may have a width dimension that issubstantially constant in the feed direction or a width dimension thatreduces in the feed direction.

The flow diversion channels may also include a second group of the flowdiversion channels that extend from the central feed channel to thewelding chamber. In an example, the flow diversion channels of thesecond group extend obliquely to the central feed channel, for exampleat an angle of 30-60 degrees relative to the extrusion direction.

The die may also be adapted to allow fabrication of hollow core fibre.This can be achieved by providing the die with a mandrel extending downthe central feed channel into the core forming conduit with a dependentpeg thereof so as to form a hollow core in the preform.

The central feed channel is advantageously connected to the core formingconduit by a taper, thereby to ensure smooth feed of material.

According to a second aspect of the invention there is provided anextruder apparatus including a main body having a location for receivingan extruder die according to the first aspect of the invention, a spacefor arranging a billet of material above the extruder die and a forcetransmitting assembly for applying pressure to the billet to drive thematerial through the extruder die.

According to a third aspect of the invention there is provided a methodof forming a preform for manufacture into an optical fibre, comprising:

applying pressure to supply a material into a central feed channel of anextruder die by pressure-induced fluid flow;

diverting a first component of the material radially outwards into awelding chamber formed within the die;

allowing a second component of the material to flow onwards from thecentral feed channel into a core forming conduit in the die; and

dispensing the material through a nozzle having an outer part in flowcommunication with the welding chamber and an inner part in flowcommunication with the core forming conduit, to respectively define anouter wall and core of the preform.

The method may use any of the die alternatives described in relation tothe first aspect of the invention.

The material supplied to the central feed channel can be a glass orpolymer. Other materials may also be contemplated.

According to a fourth aspect of the invention there is provided a methodof manufacturing an optical fibre comprising: forming a preform byextrusion according to the method of the third aspect of the invention;and reducing the preform to an optical fibre.

In some embodiments, reducing the preform to an optical fibre comprisesreducing the preform to a cane followed by reducing the cane to theoptical fibre. In that case, the preform generated directly by theextruder die can be termed a cane preform. Reducing the cane maycomprise arranging the cane in a tubular jacket and reducing the caneand tubular jacket into the optical fibre. The cane and tubular jacketmay then be referred to as a fibre preform. As an alternative toarranging the cane in a tubular jacket, reducing the cane may comprisearranging the cane amongst a plurality of rods and/or tubes to form astack and reducing the stack into the optical fibre.

In other embodiments, the optical fibre may be drawn directly from thepreform generated by the extruder die, in which case the preformgenerated directly by the extruder die will be a fibre preform (not acane preform).

According to a fifth aspect of the invention there is provided a preformfor manufacture into an optical fibre made using the method of the thirdaspect of the invention.

According to a sixth aspect of the invention there is provided anoptical fibre made using the method of the fourth aspect of theinvention.

According to a seventh aspect of the invention there is provided apreform for manufacture into an optical fibre, comprising a coresuspended in an outer wall by a plurality of struts.

The struts may have a width dimension smaller than a width dimension ofat least one of the outer wall and the core by a factor of at least two.In examples, the factor is at least one of 3, 4, 5, 6, 7, 8, 9 and 10.The struts may incorporate at least one bend in order to increase theirradial length. The wall as viewed in cross-section may deviate from acircular shape so as to provide wall sections interconnectingwall-to-strut junctions that are shorter than would be required to forma circular-section outer wall. The core may have a thickness that variesalong its axial extent. The struts may extend helically. The preform mayinclude at least one further core. The preform may include at least oneintegral electrode. The struts may have a width and a radial length andthe radial length is greater than the width. In examples, the radiallength of the struts is greater than the width by a factor of one of: 2,3, 4, 5, 6, 7, 8, 9, 10 and 20. The preform may be made of a glassmaterial, a polymer material, including a mixture of glass and polymer,such as polymer outer regions and glass central regions, including thecore.

According to an eighth aspect of the invention there is provided anoptical fibre comprising a core suspended in an outer wall by aplurality of struts.

The struts may have a width dimension smaller than a width dimension ofat least one of the outer wall and the core by a factor of at least two.In examples, the factor is at least one of 3, 4, 5, 6, 7, 8, 9 and 10.

The core may have a thickness that varies along its axial extent. Thefibre may include at least one further core, for example two cores,three cores, four cores or a higher number of cores. The struts mayextend helically. The fibre may include at least one integral electrode.The electrode material may be incorporated during extrusion, or duringsubsequent caning or drawing, or after drawing.

The struts may have a radial length greater than at least one of 2, 3,4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18 and 20 micrometers.

The struts may have a width smaller than the radial length of the strutsby a factor of at least one of 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16,18 and 20.

The optical fibre may be made of a glass material or a polymer material,including a mixture of both.

The core width may be greater than at least one of: 0.3, 0.5, 1, 2, 3,4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18 and 20 micrometers.

The core may be solid or hollow.

According to a ninth aspect of the invention there is provided a methodof manufacturing a microstructured optical fibre comprising: forming byextrusion a preform comprising a core suspended in an outer wall by aplurality of struts; and reducing the preform into an optical fibre.

According to a tenth aspect of the invention there is provided a laser,amplifier, non-linear device, switch, acousto-optic, sensor or otheroptical device comprising optical fibre according to the eighth aspectof the invention. Other devices can also be made, as described in moredetail further below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 shows in schematic cross-section a capillary stacked opticalfibre cane preform of the prior art;

FIG. 2 is a cross-sectional micrograph of an example prior artmicrostructured fibre made from a cane preform generally as shown inFIG. 1;

FIG. 3 shows in schematic cross-section an idealised microstructuredoptical fibre;

FIG. 4 shows in schematic cross-section an optical fibre made accordingto Kaiser & Astle [2];

FIG. 5 shows in schematic cross-section a disc extruder of the prior artfor making conventional optical fibre;

FIG. 6 shows in schematic cross-section a conventional optical fibremade using disc extrusion;

FIG. 7 schematically shows an extruder die used by Roeder & Egel-Hess[3] to make a glass tube;

FIGS. 8 a-d show schematic perspective views of glass structuresfabricated by Roeder & Egel-Hess [3];

FIG. 9 a shows in schematic cross-section an extruder die according toone embodiment of the invention;

FIG. 9 b shows in schematic cross-section an outer die part of theextruder die shown in FIG. 9 a;

FIG. 9 c is a side view schematically showing an inner die part of theextruder die shown in FIG. 9 a;

FIG. 9 d shows in schematic cross-section the inner die part of theextruder die shown in FIG. 9 c;

FIG. 9 e shows in schematic plan view the lower face of the extruder dieshown in FIG. 9 a;

FIG. 10 shows an exploded schematic perspective view of a lower portionof the extruder die shown in FIG. 9 a;

FIG. 11 shows in schematic cross-section an extrusion assemblycontaining the extruder die shown in FIG. 9 a;

FIG. 12 shows a schematic perspective view of an extruded cane preformmanufactured using the extrusion assembly shown in FIG. 11;

FIG. 13 a shows a schematic perspective view of the extruded canepreform of FIG. 12 within a tubular outer cladding so forming an opticalfibre preform;

FIG. 13 b shows a schematic perspective view of the extruded canepreform of FIG. 12 within a capillary stacked outer cladding;

FIG. 14 shows a schematic perspective view of an upper part of a drawingtower for drawing optical fibres;

FIG. 15 a is a photograph of a first example of a cane preformmanufactured according to a first embodiment of the invention;

FIG. 15 b is a photograph of a first example of a caned preformmanufactured according to a first embodiment of the invention;

FIG. 15 c is a scanning electron microscope image of a first example ofa drawn optical fibre according to a first embodiment of the invention;

FIG. 15 d is an optical microscope image of an alternative example of adrawn optical fibre according to a variant of the first embodiment ofthe invention;

FIG. 16 a is a contour plot which schematically shows the modelled modeshape of the optical fibre at 633 nm shown in FIG. 15 c;

FIG. 16 b is a plot which schematically shows the measured mode profileof the optical fibre shown in FIG. 15 c at 633 nm;

FIG. 17 a shows in schematic cross-section an extruder die according toa second embodiment of the invention;

FIG. 17 b shows in schematic cross-section an outer die part of theextruder die shown in FIG. 17 a;

FIG. 17 c is a side view schematically showing an inner die part of theextruder die shown in FIG. 17 a;

FIG. 17 d shows in schematic cross-section the inner die part of theextruder die shown in FIG. 17 c;

FIG. 17 e shows in schematic plan view the lower face of the extruderdie shown in FIG. 17 a;

FIG. 18 a shows in schematic cross-section an extruder die according toa third embodiment of the invention;

FIG. 18 b shows in schematic cross-section an outer die part of theextruder die shown in FIG. 18 a;

FIG. 18 c is a side view schematically showing an inner die part of theextruder die shown in FIG. 18 a;

FIG. 18 d shows in schematic cross-section the inner die part of theextruder die shown in FIG. 18 c;

FIG. 18 e is a schematic perspective view of a spider disc and mandrelassembly of the extruder die shown in FIG. 18 a;

FIG. 18 f shows in schematic plan view the lower face of the extruderdie shown in FIG. 17 a;

FIG. 18 g shows a schematic perspective view of an extruded cane preformmanufactured using the extruder die shown in FIG. 18 a;

FIG. 19 a is a side view schematically showing an inner die part of anextruder die according to a fourth embodiment of the invention;

FIG. 19 b shows in schematic plan view the lower face of an extruder dieaccording to a fifth embodiment of the invention;

FIG. 19 c shows a schematic perspective view of an extruded cane preformmanufactured using the extruder die shown in FIG. 19 b;

FIG. 19 d is a side view schematically showing an inner die part of anextruder die according to a sixth embodiment of the invention;

FIG. 20 shows in schematic plan view the lower face of several extruderdies according to further embodiments of the invention;

FIG. 21 schematically shows a 1300 nm fibre amplifier based on aPr:doped gallium lanthanum sulphide microstructured fibre;

FIG. 22 is a graph showing the Raman amplification process of a Ramanamplifier incorporating microstructured optical fibre;

FIG. 23 illustrates schematically a Brillouin laser based on a length ofmicrostructured optical fibre;

FIG. 24 schematically shows an Er:doped gallium lanthanum sulphidemicrostructured fibre laser;

FIG. 25 schematically shows a high power Nd:doped microstructured fibrelaser;

FIG. 26 schematically shows a spectral broadening device based on acompound glass microstructured fibre;

FIG. 27 schematically shows a cross-section through a microstructuredfibre for gas sensing;

FIG. 28 schematically shows a gas sensor using the fibre of FIG. 27;

FIG. 29 is an optical switch based on a gallium lanthanum sulphidemicrostructured fibre grating;

FIG. 30 is a further optical switch based on a null coupler made ofgallium lanthanum sulphide microstructured fibre;

FIG. 31 is a schematic longitudinal axial section through aforward-interaction second harmonic generator (SHG) device; and

FIG. 32 is a schematic drawing of a backward-interaction three-wavemixing (TWM) device embodying the invention.

DETAILED DESCRIPTION First Embodiment

FIG. 9 a schematically shows in vertical section an extruder die 100 foruse in manufacturing a cane preform for drawing into an optical fibreaccording to a first embodiment of the invention. In this example, theextruder die 100 is manufactured from stainless steel grade 303 which ispolished to reduce friction. In certain circumstances other materialsmay be more appropriate, for example where a higher extrusiontemperatures is preferred or different bulk mechanical properties of thedie are required. Additionally, surface coatings may be applied to thedie to assist the extruding process. The die 100 comprises an inner diepart 102 and an outer die part 104 which together define a weldingchamber 106 which opens to the lower face of the extruder die 100.

FIG. 9 b schematically shows in vertical section the outer die part 104.In this example, the outer die part 104 is cylindrically symmetric. Theexternal profile consists of a tapered cone 108 ending in a paralleldiameter 110. The inner profile consists of a parallel bore 112 ofsuitable diameter to mate with the inner die part 102 and whichterminates in a tapering step 114 and a radius edge 116 to create areduced bore profile 118.

FIG. 9 c schematically shows a side view of the inner die part 102. Inthis example, the inner die part 102 has three-fold rotational symmetryabout a central vertical axis. The external vertical face 120 of theinner die part is circular and stepped with a tapered region and endingin a parallel spigot 122 as shown in the figure. The upper face of theinner die part 102 has a concave taper 124.

FIG. 9 d schematically shows in vertical section the inner die part 102.On the centre axis of the inner die part 102 there is a first axialchannel 126 in fluid communication via a taper with a narrower secondaxial channel 128 which is in turn is in fluid communication with astill narrower third axial channel 130. The first axial channel 126 andthird axial channel 130 are respectively open to the upper and lowerfaces of the inner die part 102. The first and second 126, 128 axialchannels combine to form a central feed channel and the third axialchannel 130 forms a cane preform core forming conduit The second axialchannel 128 is in fluid communication with a group of threeequi-angularly spaced radial flow diversion channels 132 which extend tothe external face 120 of the inner die part 102. The third axial channel130 is in fluid communication with a further group of threeequi-angularly spaced radial flow diversion channels 134 defined bypairs of mutually facing internal walls and which also extend to theexternal face 120 of the inner die part 102. The radial channels 132 andthe radial channels 134 are aligned and in vertical fluid communicationwith the radial channels 134 open to the lower face of the inner diepart 102.

FIG. 9 e schematically shows a view of the lower face of the extruderdie 100 and demonstrates the openings of the third axial channel 130,the radial channels 134 and a cane preform wall forming opening 107associated with the gap formed between the reduced bore profile 118 ofthe outer die part 104 and the outer profile of the parallel spigot 122of the inner die part. The openings in the lower face of the extruderdie combine to form a nozzle for extrusion.

FIG. 10 is an exploded schematic perspective view of a lower portion ofthe die 100 and which further details the layout of the axial 126, 128,130 and radial 132, 134 channels within the inner die part 102.

FIG. 11 shows the extruder die 100 in use within an extruder dieassembly 140. The extruder die assembly 140 comprises a main body 142, apiston 144, a sleeve 146 and a cap 148. Towards the bottom of the mainbody 142 a recess is shaped to receive and locate the extruder die 100.The lower face of the extruder die assembly 140 is open as indicated inthe figure. The extruder die assembly 140 is held together by fixings152. The temperature distribution within the extruder die assembly 152is measured by a number of thermocouples (not shown) which are mountedin a plurality of thermocouple recesses 154. In operation, the extruderdie assembly 140 is loaded with a billet of glass 156 located betweenthe upper face of the extruder die 100 and the lower face of the piston144 and within a cavity formed by the sleeve 146. The sleeve 146 isremovable such that it can be easily cleaned or replaced after eachextrusion process.

The process of extrusion begins by first heating the extruder dieassembly 140 with a heater (not shown) such that the viscosity of theglass 156 is suitable for the chosen extruder die profile. Trial anderror is used to optimise the viscosity for each glass or polymer. Whenthe suitable temperature is obtained, the piston 144 is driven towardsthe extruder die 100 by an external vertically applied forceschematically indicated by the arrow. The applied force is such that theglass 156 is extruded at a suitable pressure and velocity and may, forexample, be generated by a hydraulic ram applied to the upper surface ofthe piston 144. The applied force is optimised by trial and error foreach glass or polymer. Under the application of the external force theglass 156 is forced into the extruder die 100. The glass 156 fills thevolume defined by the concave taper 124 and is further forced into thefirst axial channel 126 and subsequently along a feed direction into thesecond axial channel 128. A component of the glass 156 from the secondaxial channel 128 is forced onward into the third axial channel 130,whereas a second component is diverted radially by the radial channels132 to fill the welding chamber 106. The separate glass streams enteringthe welding chamber 106 from the three of the radial channels 132 expandcircumferentially within the welding chamber 106 and re-weld into asingle continuous tubular form. A combination of glass 156 from thewelding chamber 106, the radial channels 132 and the third axial channel130 is further urged to fill the radial channels 134.

At this stage of the extrusion process, the air spaces within theextruder die 100 are filled with glass and under continued applicationof the pressure inducing force, glass begins to be extruded from thenozzle of the extruder die 100. The glass is extruded in a pattern whichis determined by the openings in the lower face of the extruder die 100indicated in FIG. 9 e.

FIG. 12 is a schematic perspective view of a glass cane preform 160obtained from the extruder die assembly 140. The preform 160 comprisesan outer wall 162 of tubular form and with a wall thickness W_(j) andouter diameter D_(j), a central core 164 of circular cross-section anddiameter D_(c) and three linear radial struts 166 of width W_(s) andlength L_(s). The cane preform 160 has an overall length of L. The outerwall 162 is created by glass extruded through the opening 107 in thelower face of the extruder die 100 defined by the gap between the innerdie part 102 and the outer die part 104. Its dimensions are accordinglydetermined by those of the outer diameter of the parallel spigot 122 andthe inner diameter of the reduced bore profile 118. The central core 164is created by the opening of the third axial channel 130 in the lowerface of the extruder die 100 and its diameter accordingly determined bythat of the channel 130. The struts 166 are created by the opening ofthe of radial channels 134 in the lower face of the extruder die 100 andtheir dimensions accordingly determined by the horizontal cross-sectionof these channels 134.

The cane preform 160 is especially suited for fabricating an opticalfibre in which the central core 164 becomes a light guiding coresupported within the drawn wall 162 by the drawn struts 166. Unlikeprevious die designs, the central core 164 formed by the extruder die100 comprises glass which has not undergone splitting into separatestreams and re-welding within the die. This is important for maintaininghigh optical integrity of the glass in the core region of the drawnfibre. As noted by Roeder & Egel-Hess, the re-welded glass of prior artextruder dies does not provide extrusions suitable for opticalapplications. The present die design further allows the cross-section ofthe cane preform 160 to display a wide range of wall thicknesses. Thisis achieved by lowering surface friction in some areas by reducing thepath length of the flowing glass within various channels, and injectinggreater volumes of glass into regions requiring greater wall thickness.For example, wall 162 width W_(j) to strut 166 width W_(s) ratios of5.4:1, 12:1 and 15:1 have been achieved. The strut 166 length L_(s) canalso be several times longer than the strut 166 width W_(s). Strutlength W_(j) to strut width W_(s) ratios of 5:1 and 12.5:1 have beenprepared in specific examples.

The first stage of drawing the cane preform 160 into an optical fibre iscaning. The extruded cane preform outer diameter D_(j) might typicallybe around 10-30 mm. The cane preform 160 is caned down to produce a canewhich has a diameter around ten times smaller than the cane preform 160,the caning can, for example, be done in a drawing tower. In the processof pulling the cane preform into the cane (or even directly into afibre), it can be desirable to seal the end of the cane preform oralternatively to actively pressurise the structure relative to theexternal environment in order to help to prevent collapsing during thedraw due to surface tension effects. The cane is then further drawn toprovide a suitably sized guiding core. To provide sufficient structuralrigidity, a supporting cladding region is generally applied to the caneto provide a fibre preform for drawing.

FIG. 13 a is a schematic perspective view a fibre preform 170 which isto be drawn to form an optical fibre. The fibre preform 170 comprises acane 171 made from the preform 160 provided by the extruder die assembly140 and a supporting tube 172. The cane 171 is placed within thesupporting tube 172 to form the fibre preform 170. The inner diameter ofthe supporting tube 172 closely matches the outer diameter of the cane171. The outer diameter of the supporting tube 172 is chosen to suit thedesired outer geometry of the fibre to be drawn. The supporting tube 172may be manufactured by any suitable means, including extrusion. Thesupporting tube 172 may preferentially be made of the same material asthe cane 171 to ensure mechanical and thermal compatibility. However, ifa specialist glass is used for the original preform 160, it may be moreappropriate for the supporting tube 172 to be of a different suitablematerial.

During drawing, it can be advantageous to apply a vacuum to the spacebetween the outside of the cane and the inside of the supporting tube.This inhibits contraction of the cane and generates a force that acts toclose the space. As a result, during drawing of fibre, the outer wall ofthe cane bonds with the inner wall of the supporting tube to form asingle structure.

FIG. 13 b is a schematic perspective view of an alternative fibrepreform structure 174 which could be drawn into an optical fibre. Thecane 171 is incorporated within a structured surround comprising ahexagonally packed array of tubes and/or rods 175, 176. In this example,the cane 171 is surrounded by a first ring of glass tubes 175 and twofurther rings of solid glass rods 176. In another example, the solidrods may be replaced with tubes. The assembly is held together by aglass outer jacket 177. As with the support cladding 172 shown in FIG.13 a, some or all of the structured surround components 175, 176, 177may be made of the same glass 156 as the cane 171. The tubes 175 may beparticularly useful for incorporating electrodes for thermally polingthe drawn fibre. The electrodes can be created by inserting metal wires(e.g. gold or tungsten) into the holes in one or more tubes 175 beforecaning or drawing. Electrodes may also be located interstitially withrespect to the lattice formed by the tubes 175 and/or rods 176 whichform the support cladding region. Instead of using metal wires, theelectrodes could also be drawn from graphite, graphite alloy or graphitedoped rods. Other conductive materials or dopants may also be used.Alternatively, the electrodes may be inserted into the holes after fibredrawing.

A still further alternative would be to extrude a preform withsufficiently large outer diameter D_(j) that no further cladding isrequired. Such a preform has even fewer glass-glass or air-glassinterfaces which are often a source of contamination in optical fibres.A preform with an outer diameter which is large enough to remove theneed for further cladding may require multiple caning and or drawingstages to provide suitable drawn fibre dimension or may be drawndirectly into a fibre.

FIG. 14 shows a furnace used to draw a fibre preform into an opticalfibre. In the process of pulling the fibre preform it is typicallysealed at the top (where the bottom is defined as the portion that willbe fed through the furnace first). This is in order that the holes inthe cross-sectional structure of the cane do not collapse during fibredrawing. This could also potentially be achieved by setting anover-pressure for the holes that define the cross-sectional structure(relative to the outside pressure). Another approach, that could be usedeither on its own or in conjunction with the above mentioned methodswould be to evacuate the space between the cane and the supportingjacket that surrounds the cane in the fibre preform during the fibrepulling process. This provides a pressure differential during the drawprocess, which should keep the holes in the caned preform open whilstclosing up any undesirable gaps between the support jacket and themicrostructured cane. These techniques can also be applied to the voidswithin a structured support jacket of the fibre preform (such as withintubes, or located interstitially between rods and/or tubes forming thestructured support jacket) which can be encouraged to either close up orremain open as desired during drawing. The furnace incorporates aninductively heated (RF) hot zone defined by water-cooled helically woundRF coils 180. In use, the water cooled RF coils generate an RF fieldthat heats a graphite susceptor (not visible). In the illustratedfurnace, the RF coils define a 50 mm long hot zone around and along thefibre preform.

A combination of water and gas cooling is provided above and below thehot zone. The cooling keeps the material outside the hot zone cooled tobelow its crystallisation temperature. Elements of the cooling systemare apparent from the figure, namely an upper gas halo 182, a lower gashalo 184, a cold finger 186, and a water jacket 188 made of silica. Theupper gas halo and silica water jacket cool the fibre preform prior toentry into the hot zone. The cold finger, and lower gas halo providerapid cooling after the fibre emerges from the hot zone. A thermocouple190 for monitoring furnace temperature is also indicated. Thethermocouple forms part of a control system for regulating the furnacetemperature.

Other furnace types are also suitable, for example based on resistiveheating such as a graphite resistance furnace.

A range of different coating materials can be used for coating theoutside of a fibre preform prior to or during drawing. Examples ofcoating materials are standard acrylates, resin, Teflon (trade mark),silicone rubber, epoxy or graphite. In particular, graphite coating canbe used to good effect since it promotes stripping of cladding modes andalso provides enhanced mechanical strength.

Depending on the desired final geometry and the geometry of the cane,multiple stages of drawing may be necessary.

First Embodiment: Example

FIG. 15 a is a photograph showing an extruded cane preform 160 which hasbeen fabricated using an extruder die 100 according to the firstembodiment of the invention described above.

The cane preform 160 is made from SF57 glass, a commercially availableSchott glass. The high lead concentration of this glass leads to a highrefractive index of 1.83 at 633 nm and 1.80 at 1.53 μm with losses inthe bulk glass of 0.7 dB/m at 633 nm and 0.3 dB/m at 1.53 μm. Thenon-linear refractive index (n₂) measured at 1.06 μm is 4.110⁻¹⁹ W²/m[4], more than an order of magnitude larger than that of pure silicaglass fibres [5]. Since the effective non-linearity of a fibre isγ=n₂/A_(eff), where A_(eff) is the effective mode area. The combinationof this glass with the small effective areas (A_(eff)) possible inmicro-structured fibres allows for dramatic improvements in thenon-linearity that can be achieved.

SF57 glass has a low softening temperature (519° C.). The cane preform160 was extruded from bulk SF57 glass. A cross-section through theextruded cane preform 160 has an outer diameter (OD) of 16.5 mm, strutthickness 0.375 mm, strut length 5.65 mm, preform length about 10 cm andcore diameter 1.2 mm. As described above, and as seen in FIG. 15 a, thecane preform is comprised of a central core 162 supported by three longstruts 166. This transverse structure extends along the entire canepreform length L.

FIG. 15 b is a photograph showing a cane 171 created by caning theextruded cane preform 160 shown in FIG. 15 a down to an OD of 1.6 mmwith the other dimensions reducing roughly to scale. It is evident thatthe cross-sectional shape of the cane preform 160 is well maintained inthe cane 171. The cane 171 is inserted within an extruded jacketing tube172, as schematically shown in FIG. 13 a, and the resulting fibrepreform is drawn down to 120 μm OD optical fibre.

FIG. 15 c is a scanning electron microscope image of an optical fibre192 drawn from the fibre preform 170 described above. In this process,extremely small features have been retained within the final fibre 192without compromising practicality and handling.

Visual inspection of the drawn fibre 192 indicates that thiscross-sectional profile remained essentially unchanged over more than 50m of the fibre. The central core diameter in this example drawn fibre is2 μm and the central core is suspended by three 2 μm long struts thatare less than 400 nm thick. The supporting struts allow the solidcentral core to guide light by helping to isolate the central core fromthe outer solid regions of the fibre cross-section.

In FIG. 15 c three elongate cross-section holes are evident outside thecore and strut structure. These holes have formed because of partialcollapse of the cane during drawing. This can be prevented by applyingvacuum suction between the cane and supporting tube during drawing, asmentioned above in relation to FIG. 13 a

FIG. 15 d shows a holey fibre drawn using a vacuum in this way. As isevident there are no outer elongate holes, the gap between the outsideof the cane and the supporting tube having been closed during drawing.

FIG. 16 a is a contour plot showing the predicted mode profile at 633 nmin the xy plane (defined to be perpendicular to the longitudinal axis ofthe fibre) of the fibre 192 as a function of position x,y from thecentral axis of the fibre, individual contours are separated by 1 dB.Measurements taken from the scanning electron microscope image shown inFIG. 15 c are used to define the transverse structure and an efficientmodal model [6] used to predict the properties of the fibre at 633 μm.In FIG. 16 a the predicted mode profile shown is superimposed on thegeometry of the core region. The effective mode area is A_(eff)=2 μm²,comparable to the smallest areas achieved in silica microstructuredfibres. Hence these SF57 fibres offer values of the effectivenon-linearity γ that are three orders of magnitude higher thanconventional silica optical fibres.

FIG. 16 b is a graph showing an experimentally determined mode profilefor the fibre 192 at 633 nm and shows the intensity I as a function ofradial distance x from the central axis of the fibre 192. Robustsingle-mode guidance was observed in the fibre at both 633 nm and 1500nm.

Although single-material fibres support only leaky modes, it is possibleto design low-loss fibres of the type shown in FIG. 15 c [6]. This canbe done by ensuring that the supporting struts are long and fine enoughthat they act purely as structural members that isolate the core fromthe external environment. In the final fibre, the struts may have radiallengths of at least 2 micrometers, up to 20 micrometers or longer. Thestrut widths will generally be smaller than the radial length by afactor of at least 2 and as much as 10 or 20 or more.

The fibres can be effectively single-mode over a broad range ofwavelengths since the confinement losses associated with any higherorder modes are significantly higher than that of the fundamental mode.Note that confinement losses typically increase with wavelength.

Another design option is to make the struts with variablecross-sectional thickness. For example, the struts may be thicker ateither end (at the core end and outer wall end) and thinner in themiddle, incorporating a smooth inward and outward taper. A single taperfrom thin at the core to thick at the outer wall, or vice versa couldalso be implemented. This could, for example, alter the structuralproperties of the fibre without significantly effecting the opticalproperties of the fibre.

We observe approximately 3 dB/m loss at 633 nm and 10 dB/m at 1550 nm,significantly larger than the material loss at each wavelength. Weanticipate that the confinement loss would decrease significantly whenstill longer struts are used. The strut length in the fibre in FIG. 15 cwas not limited by the extrusion process, as FIGS. 15 a and 15 b attest,and so we anticipate further improvements.

Second Embodiment

FIG. 17 a schematically shows in vertical section an extruder die 200for use in manufacturing an optical fibre preform according to a secondembodiment of the invention. This particular embodiment is designed toproduce a cane preform with greater cross-sectional outer wallthicknesses. In this example, the extruder die 200 is again manufacturedfrom stainless steel grade 303, and is polished to reduce friction. Thedie 200 comprises an inner die part 202 and an outer die part 204 whichtogether define a welding chamber 206 which is in fluid communicationwith an opening to the lower face of the extruder die 200.

FIG. 17 b schematically shows in vertical section the outer die part204. In this example, the outer die part 204 is cylindrically symmetric.The external profile consists of a tapered cone 208 ending in a paralleldiameter 210. The inner profile consists of a parallel bore 212 ofsuitable diameter to mate with the inner die part 202 and whichterminates in a tapering step 214 and a radius edge 216 to create areduced bore profile 218.

FIG. 17 c schematically shows a side view of the inner die part 202. Inthis example, the inner die part has three-fold rotational symmetry. Thevertical external face 220 of the inner die part is circular and steppedwith a tapered region and ending in a parallel spigot 222 as shown inthe figure.

FIG. 17 d schematically shows in vertical section the inner die part202. On the centre axis of the inner die part 202 there is a first axialchannel 226 in fluid communication via a taper with a narrower secondaxial channel 228 which is in turn is in fluid communication with astill narrower third axial channel 230. The first axial channel 226 andthird axial channel 230 are respectively open to the upper and lowerfaces of the inner die part 202. The first and second axial channels226, 228 combine to form a central feed channel and the third axialchannel 230 forms a cane preform core forming conduit. The first andsecond axial channels 226, 228 are in fluid communication with a groupof three equi-angularly spaced radial flow diversion channels 232 whichextend to the external face 220 of the inner die part 202. The thirdaxial channel 230 is in fluid communication with a further group ofthree equi-angularly spaced radial flow diversion channels 234 definedby pairs of mutually facing internal walls and which also extend to theexternal face 220 of the inner die part 202. The radial channels 232 andthe radial channels 234 are aligned and in vertical fluid communicationwith the group of radial channels 234 open to the lower face of theinner die part 202. The first axial channel 226 is also in fluidcommunication with a still further group of three equi-angularly spacedradial channels 233 which extend obliquely to the external face 220 ofthe inner die part 202. The channels 233 are angularly inter-spacedbetween the radial channels 232 and angled downwards along a radiallyoutward direction as indicated in FIG. 17 d.

FIG. 17 e schematically shows a view of the lower face of the inner diepart 202 and demonstrates the openings of the third axial channel 230and the radial channels 234. The projected opening of the radialchannels 232,233 are also shown.

The operation of the die 200 in a glass extrusion process will besimilar to and understood from the description given above withreference to the first embodiment. However, in the die 200, the combinedincreased flow capacity of the radial channels 232, 233 (both becausethe radial channels 232 are of relatively longer extent along the feeddirection than in the first embodiment and the group of radial channels233 are additional) allow the welding chamber 206 to be relativelylarger than the welding chamber 106 of the first embodiment. Sincerelatively more glass is diverted to the relatively large weldingchamber 206, thicker walls can be efficiently extruded from the die 200.

Third Embodiment

FIG. 18 a schematically shows in vertical section an extruder die 800for use in manufacturing an optical fibre preform according to a thirdembodiment of the invention. This particular embodiment is designed toproduce a cane preform in which the central core is hollow. In thisexample, the extruder die 800 is again manufactured from stainless steelgrade 303, and is polished to reduce friction. The die 800 comprises aninner die part 802 and an outer die part 804 which together define awelding chamber 806 which is in fluid communication with an opening tothe lower face of the extruder die. The extruder die 800 furthercomprises a spider disc 805 and a mandrel 803.

FIG. 18 b schematically shows in vertical section the outer die part804. In this example, the outer die part 804 is cylindrically symmetric.The external profile consists of a tapered cone 808 ending in a paralleldiameter 810. The inner profile consists of a parallel bore 812 ofsuitable diameter to mate with the inner die part 802 (as shown in FIG.18 a) and which terminates in a tapering step 814 and a radius edge 816to create a reduced bore profile 818.

FIG. 18 c schematically shows a side view of the inner die part 802. Inthis example, the inner die part has three-fold rotational symmetry. Thevertical external face 820 of the inner die part is circular and steppedwith a tapered region and ending in a parallel spigot 822 as shown inthe figure.

FIG. 18 d schematically shows in vertical section the inner die part802. On the centre axis of the inner die part 802 there is a centralfeed channel made up of a first axial channel 826 in fluid communicationvia a taper with a narrower second axial channel 828. The second axialchannel 828 is in turn in fluid communication with a still narrowerthird axial channel 830 that forms the core forming conduit. The outerdiameter of the first axial channel 830 changes from a first value to asecond value to define a stepped recess 827 as indicated in the figure.The first axial channel 826 and third axial channel 830 are respectivelyopen to the upper and lower faces of the inner die part 802. The firstand second axial channels 826, 828 are in fluid communication with athree equi-angularly spaced radial flow diversion channels 832 whichextend to the external face 820 of the inner die part 802. The thirdaxial channel 830 is in fluid communication with a further threeequi-angularly spaced radial flow diversion channels 834 defined bypairs of mutually facing internal walls and which also extend to theexternal face 820 of the inner die part 802. The radial channels 832 andthe radial channels 834 are aligned and in vertical fluid communication.The radial channels 834 are further open to the lower face of the innerdie part 802. The first axial channel 826 is also in fluid communicationwith a still further group of three equi-angularly spaced radialchannels 833 which extend obliquely to the external face 820 of theinner die part 802. The radial channels 833 are angularly inter-spacedbetween the radial channels 832 and angled downwards along a radiallyoutward direction as indicated by their projected appearance marked onthe vertical section drawing shown in FIG. 18 d.

FIG. 18 e is a schematic perspective view showing the assembled spiderdisc 805 and mandrel 803. The spider disc 805 has the form of a flatcircular disc with a plurality of holes 880, 881. A first central hole880 is tapped and able to receive and hold the mandrel 803 centrally in,and extending perpendicularly to, the spider disc 805. In this example,the mandrel 803 is a circularly symmetric with a threaded upper part(not shown) for affixing the mandrel into the tapped hole 880. The outerprofile of the mandrel has the form of a cylindrical section of a firstdiameter and which tapers down to a cylindrical section of a secondsmaller diameter at its distal end to form a downwardly depending peg807 which sleeves into the core forming conduit 830. The remaining holes881, of which in this example there are three, are radially displacedfrom the central axis of the spider disc and allow fluid communicationbetween the upper and lower circular faces of the spider disc. The outerdiameter of the spider disc matches the outer diameter of the upper partof the first axial channel 826 such that in operation the spider disc805 is restrained and seated within the recess 827. With the spider disc805 seated within the inner die part 802, the mandrel 803 extendscentrally along the first, second and third axial channels. The outerdimensions of the mandrel 803 are such that it is able to pass freelythrough the axial channels whilst a fluid communication path between theaxial channels is maintained. The length of the mandrel 803 is such thatit extends throughout the inner die part 802 and terminates with the endof the peg 807 at or around its lower face.

FIG. 18 f schematically shows a view of the lower face of the inner diepart 802 and demonstrates the openings of the third axial channel 830and the radial channels 834. The projected openings of the radialchannels 832 and 833 and the end of the mandrel 803 are also shown.

In operation, the die 800 is mounted in a die extruder assembly which issimilar to and will be understood from that shown in FIG. 11 inconnection with the first embodiment. However, during extrusion theglass flow pattern within the body of the die is slightly different tothat of the first embodiment. Under application of the extruding force,the glass is forced through the holes 881 in the spider disc 805 andreforms within the first axial channel 826 in the space surrounding themandrel 803. The glass flow from this channel to the radial channels 832and 834 and to the welding chamber 806 is similar to and will beunderstood from the description given above in connection with thesecond embodiment. However, the component of glass which passes alongthe second and third axial channels is now only able to pass between theouter diameter of the mandrel 803 and its peg 807 and the inner diameterof second and third axial channels 828 and 830. Accordingly, theeffective core forming conduit formed by the axial channels and themandrel has the cross-sectional form of an annular ring.

FIG. 18 g is a schematic perspective view of a portion of a glass canepreform 860 obtained from the extruder die 800. The preform 860comprises an outer wall 862 of tubular cross-section and three linearradial struts 866. These are formed in a manner which is similar to andwill be understood from the corresponding features shown in FIG. 12.However, the central core 864 is different to that shown in FIG. 12. Thecore 864 is created by the gap surrounding the mandrel 803 within theopening of the third axial channel 830 in the lower face of the extruderdie 800 and as such has a tubular cross-section as indicated in thefigure. A fibre drawn from such a cane preform may, for example, supporta ring mode. The hollow core may also be filled, for example, a secondglass rod could be inserted into the hollow core of the cane preformprior to caning or drawing to provide a drawn fibre with different coreglasses. Furthermore, the mandrel need not have a circularcross-section. An oval cross section could be used to produce a canepreform with a hollow core having a circular outer profile but an ovalinner profile. In constructing a fibre preform from such a cane preform,in addition to a supporting jacket such as indicated in FIGS. 13 a and13 b, the central hollow core may be filled prior to drawing. Forexample, a central cylindrical glass rod and two diametrically oppositewires could be inserted to allow poling of a small central core within adrawn fibre.

It will also be understood that other dies may be designed using theseprinciples for making preforms with multiple hollow cores, or a mixtureof hollow cores and solid cores wherein the cores may be located axiallyor parallel thereto displaced from the principal die axis.

Fourth Embodiment

FIG. 19 a schematically shows a side view of an inner die part 302 of adie according to a fourth embodiment of the invention. In operation, theinner die part 302 would combine with an outer die part which is notshown, but which would be similar to and understood from the descriptionof the outer die part 104 of the first embodiment. In this example, theinner die part has four-fold rotational symmetry. The vertical externalface 320 of the inner die part is circular and stepped with a taperedregion and ending in a parallel spigot 322 as shown in the figure. Onthe centre axis of the inner die part 302 there is a first axial channel326 in fluid communication via a taper with a narrower second axialchannel (not shown) which is in turn is in fluid communication with astill narrower third axial channel 330. The first axial channel 326 andthird axial channel 330 are respectively open to the upper and lowerfaces of the inner die part 302. The first 326 and second axial channelscombine to form a central feed channel and the third axial channel 130forms a cane preform core forming conduit. The first 326, second andthird 330 axial channels are in fluid communication with a group of fourequi-angularly spaced radial channels 332 which extend to the externalface 320 of the inner die part 302. The cross-section of the radialchannels 332 in a plane perpendicular to the diverted flow direction isinverse teardrop shaped with the bottom end open to the lower face ofthe inner die part 302, as shown in FIG. 19 a. As glass is forcedthrough the inner die part 302 during extrusion, the upper, wider partsof the radial channels 332 allow sufficient glass flow to fill a weldingchamber formed by the inner die part 302 and the outer die part (notshown) to provide a thick outer wall for a cane preform, while thethinner openings of the radial channels 332 in the lower face of theinner die part 302 directly provide an extrusion path for forming aplurality of struts for supporting a central core in the cane preform.

Fifth Embodiment

FIG. 19 b schematically shows a plan view of a lower face (i.e. thatwhich defines the extrusion cross-section) of an extruder die 400according to a fifth embodiment of the invention.

The die 400 comprises an inner die part 402 and an outer die part 404which combine to form a welding chamber in a manner which is similar toand will be understood from the description given above for the firstembodiment. The outer profile of the inner opening on the lower face theouter die part 404 and the outer profile on the lower face of the innerdie part 402 are of a rounded-triangular form with their verticesco-aligned as indicated in the figure. A central axial opening 430 is influid communication with a wall forming opening 407 (formed by the gapbetween the outer profile of the inner die part 402 and the innerprofile of the outer die part 404 at the lower face of the die) via agroup of three radial channels 434 formed by pairs of mutually facinginternal walls. The radial channels 434 each contain a bend andintersect the wall forming opening 407 at the vertices of therounded-triangle which describes its shape. Other than the shape of theopenings in the lower face, the extruder die 400 will be functionallysimilar to and understood from the description given above for the firstembodiment. The radial channels 434 and the fluid communication pathbetween the wall forming opening 407 and the welding chamber maymaintain their curved structure within the body of the extruder die 400or may adopt it only towards the lower face.

FIG. 19 c schematically shows a perspective view of a glass cane preform460 extruded from the extruder die 400 shown in FIG. 19 b. The canepreform 460 comprises a tubular outer wall 462 of rounded-trianglecross-section, a cylindrical central core 464 and bent/curved radialstruts 466. The difference in the cross-sectional geometry of the canepreform 460 shown in FIG. 19 c compared to the cane preform 160 shown inFIG. 12 helps to provide a circular cross-section in the drawn fibre. Asseen in FIGS. 15 a, 15 b and 15 c, the caning and drawing of the canepreform 160 of the first embodiment maintains the cross-sectionalgeometry well. There is, however, a level of azimuthal distortion causedby non-uniform contraction of the outer wall 162 and central core 164due to the surface tension of the struts 166 during caning and drawing.The cane 171 (see FIG. 15 b) and the final fibre 192 (see FIG. 15 c)have slightly triangular cross sections.

The triangular cross-sectional geometry and bent struts 466 of the canepreform 460 extruded from the extruder die 400 reduces the effect on acane and final fibre of the distortive pulling by the struts during thecaning and drawing in two ways. Firstly, since the struts 466 areover-long to be purely radial, when they contract in length duringcaning and drawing, rather than pulling on the outer wall 462 andcentral core 464, they simply become less curved. Secondly, any residualpulling by the struts 466 on the outer wall 462 during caning anddrawing will act at the vertices of the rounded-triangle defining thecross-sectional shape of the tubular wall 462 and so pull the caned anddrawn wall 462 into a more circular form. Whilst the extruder die 400shown in FIG. 19 b makes use of both of these effects, each could beused independently. Other extruder die opening profiles may be used tocounteract other effects of the strut contraction during drawing. Forexample, the central core opening may also be triangular with the radialchannel openings in the lower face of the extruder die meeting thetriangular central core in the middle of each of its sides. This wouldhelp to provide a circular core in the drawn fibre if desired.

Whilst the above described measures to counteract the effects of strutcontraction during caning and drawing have concentrated on extruder diesand preforms of three-fold symmetry, they are equally applicable toother designs by choosing correspondingly appropriate outer wall and/orcentral core shapes. For example, with four-fold symmetry the outer wallshould have a rounded-square cross-section, for two fold-symmetry anoval outer wall will be preferred. Furthermore, if an asymmetric finalfibre is required, perhaps to provide a fibre with polarisationdependent losses or birefringence, the pulling effect of the strutscould be used advantageously whereby a non-circular outer wall isprovided with radial struts which meet it at locations where it isalready nearer to the central core.

Sixth Embodiment

FIG. 19 d schematically shows a side view of an inner die part 302 of adie according to a sixth embodiment of the invention. In operation, theinner die part 302 would combine with an outer die part which is notshown, but which would be similar to and understood from the descriptionof the outer die part 104 of the first embodiment. The inner dieincorporates two modifications from the design of the first embodiment.

First, the radial flow diversion channels 632 are provided with bridges629. This adds structural strength to make the die more resistant tobeing prized apart by the force of the material during extrusion. Thisis beneficial when extruding higher viscosity glasses, such as galliumlanthanum sulphide (GLS). In this example the channels 632 taper incross-section towards the output end, but bridges could be used in anon-tapered design, such as in the first embodiment.

Second, the main material feed is through a smooth tapered axial channel625 until the end where a short straight axial channel 630 is provided.The axial channel 625 narrows gradually without the steps of theprevious embodiments. This will assist a smooth increase in the pressureprofile in the feed direction. A smooth taper of this kind can bemanufactured by spark erosion.

Further Embodiments

FIG. 20 schematically shows plan views of the lower faces (i.e. thosewhich define the extrusion cross-section) of a plurality of extruderdies according to further embodiments of the invention. As will beunderstood from the following, the core may have a wide variety ofshapes, circular, polygonal etc. and the struts can have a wide varietyof lengths and thicknesses, with the thicknesses being substantiallyconstant along the strut radial length in some examples, and of varyingthickness in other examples.

The extruder die of FIG. 20 i provides a cane preform substantially asdescribed above with reference to the first embodiment of the invention.

The extruder die of FIG. 20 ii provides a cane preform with a tubularcircular outer wall and three radial struts. Each radial strut supportsa cylindrical core displaced from the central cane preform axis.

The extruder die of FIG. 20 iii provides a cane preform with a tubularcircular outer wall, a solid central core and three radial struts. Inthis example, the radial struts are not equi-angularly spaced.

The extruder die of FIG. 20 iv provides a cane preform with a tubularcircular outer wall, a solid central core and three radial struts. Inthis example, the central core has an asymmetric diamond cross-section

The extruder die of FIG. 20 v provides a cane preform with a tubularrounded-triangle outer wall, a solid central core and three radialstruts.

The extruder die of FIG. 20 vi provides a cane preform with a tubularrounded-triangle outer wall, a central core and three radial struts. Inthis example, the central core is hollow.

The extruder die of FIG. 20 vii provides a cane preform with a tubularrounded-triangle outer wall, a solid central core and three radialstruts. In this example, the radial struts are curved and meet thecentral core at the vertices of its triangular cross-section.

The extruder die of FIG. 20 viii provides a cane preform with a tubularrounded-triangle outer wall, a solid central core and three radialstruts. In this example, the radial struts are curved and meet thecentral core at the vertices of its triangular cross-section. Eachcurved radial strut also supports a cylindrical core displaced from thecentral cane preform axis.

The extruder die of FIG. 20 ix provides a cane preform with a tubularcircular outer wall, a solid central core and four radial struts. Inthis example, the central core has an elongated diamond cross-section.

The extruder die of FIG. 20 x provides a cane preform with a tubularcircular outer wall, a solid central core and four radial struts.

The extruder die of FIG. 20 xi provides a cane preform with a tubularrounded-square outer wall, a solid central core and four radial struts.

The extruder die of FIG. 20 xii provides a cane preform with a tubularcircular outer wall, a solid central core and six radial struts. In thisexample, the extruder die has six-fold symmetry.

The extruder die of FIG. 20 xiii provides a cane preform with a tubularrounded-hexagon outer wall, a solid central core and six radial struts.

The extruder die of FIG. 20 xiv provides a cane preform with a tubularcircular outer wall and a solid central core. In this example, the solidcentral core is suspended by thin struts between two hollow cores, eachof which is in turn suspended by two further thin struts to connect themto the wall. These hollow cores could, for example, incorporateelectrodes to allow for electrical poling.

The extruder die of FIG. 20 xv provides a cane preform with a tubularcircular outer wall. In this example, two solid cores are symmetricallydisposed about the central axis and are supported by a network ofstruts.

None of the cross-sectional profiles of cane preforms which could beextruded from the dies shown in FIG. 20 could be made using conventionalcapillary stacking techniques. There is an essentially limitless rangeof other profiles which could also be used. Some of these, for example,might incorporate combinations of the features shown in FIG. 20 indifferent ways. For example, the three off-axis cores provided by thedie shown in FIG. 20 ii could be combined with the four-fold symmetricalarrangement indicated in FIG. 20 x to provide a die for extruding a canepreform with four off-axis cores, with or without a central core.

While the specific details of the geometry of the opening face of theextruder die are different for each of the different cane preformprofiles, the die design principles described above are applicable toall. For example, the die design represented in FIG. 20 iv would be asdescribed with respect to the first embodiment given above, but with anon-axially symmetric third axial channel opening into the lower face ofthe die. In the die design shown in FIG. 20 ii, the third axial channelof the first embodiment is reduced to a diameter matching the thicknessof the lower group of radial channels and so no central core is formedand at the centre of the opening of each of the lower group of radialchannels a circular widening in the profile provides for the off axiscores shown in the figure. This widening may persist verticallythroughout the radial channels, or may only open up towards the lowerface of the die. The multiple cores again comprise un-re-welded glassfrom the central axis feed and so maintain high optical integrity. Inthe case of the hollow cores shown in FIG. 20 xiv, these may be providedmerely to provide ducts for electrode insertion, or may be opticallyactive, for example dimensioned to support a ring mode.

The cane preforms shown in FIGS. 12 and 19 c, have been uniformlyextruded and display constant transverse cross-sections along theirlength. In some circumstance, however, a longitudinally varying canepreform may be preferred to provide a drawn fibre in which itsproperties which vary along its length. The longitudinal non-uniformitycan be introduced in several ways. For example, a helical twist could begenerated in a cane preform by rotating it about its longitudinal axisduring extrusion. A fibre drawn from such a preform would have helicallyevolving struts and may be used, for example, to control circularbirefringence. Helically evolving struts could similarly be introducedat other stages of fibre manufacture, for example, by rotating the canepreform and/or fibre preform during a caning or drawing process. Thiswould allow higher helix pitch angles to be generated into the finalfibre. A longitudinal non-uniformity can further be introduced byvarying the rate of extrusion, for example by modifying the extrusionpressure or temperature to alter the cane preform core thickness. Thiscan be done in a continuous, cyclical or pulsed manner to respectivelycreate tapered, periodic or discretised longitudinal variations in afinal drawn fibre. These variations can also be introduced at otherstages of fibre production, for example by varying the rate at whichcaning or drawing is performed. Such longitudinal structuring can assistin dispersion management, Brillouin suppression, etc.

Materials Considerations

As described in the example above, the extruder die is made fromstainless steel grade 303. This die has been used to extrude SF57 glass.The inventors have also successfully extruded a range of other glasses,such as a tellurite glass, and a gallium lanthanum sulphide glass. Moregenerally, the invention is applicable to a wide range of glasses andnon-glasses such as polymers from which optical fibres may be made.Further examples may relate to the following glasses:

Lead glasses (e.g. SF57, SF59)

Chalcogenides (e.g. S, Se or Te-based glasses);

Sulphides (e.g. Ge:S, As:S, Ge:Ga:S, Ge:Ga:La:S);

Oxy Sulphides (e.g. Ga:La:O:S);

Halides (e.g. ZBLAN (trade mark), ALF);

Chalcohalides (e.g. Sb:S:Br);

Heavy Metal Oxides (e.g. PbO, ZnO, TeO₂);

Silicates (e.g. silicate, phosphosilicate, germanosilicate); and

Polymers (e.g. polyacrylate, polycarbonate, polystyrene, polypropylene,polyester, PMMA, Cytop (trade mark), Teflon (trade mark)).

Some specific examples are now further detailed.

In the case of a sulphide glass, this may be formed from the sulphidesof metals selected from the group: sodium, aluminium, potassium,calcium, gallium, germanium, arsenic, selenium, strontium, yttrium,antimony, indium, zinc, barium, lanthanum, tellurium and tin.

In the case of a glass based on gallium sulphide and lanthanum sulphide,glass modifiers may be used based on at least one of: oxides, halides orsulphides of metals selected from the group: sodium, aluminium,potassium, calcium, gallium, germanium, arsenic, selenium, strontium,yttrium, antimony, indium, zinc, barium, lanthanum, tellurium and tin.

In the case of a halide glass, it may be formed from fluorides of atleast one of: zirconium, barium and lanthanum. Further, glass modifiersmay be used selected from the fluorides of the group: sodium, aluminium,potassium, calcium, gallium, germanium, arsenic, selenium, strontium,yttrium, antimony, indium, zinc, barium, lanthanum, tellurium and tin.

In the case of a heavy metal oxide glass, the oxides may be selectedfrom: sodium, aluminium, potassium, calcium, gallium, germanium,arsenic, selenium, strontium, yttrium, antimony, indium, zinc, barium,lanthanum, tellurium and tin.

In the case of a heavy metal oxyfluoride glass, the glass may be formedby heavy metal oxides selected from oxides of metals of the group:sodium, aluminium, potassium, calcium, gallium, germanium, arsenic,selenium, strontium, yttrium, antimony, indium, zinc, barium, lanthanum,tellurium and tin and 0-50 mol % total fluoride.

In the case of a heavy metal oxychloride glass, the glass may be formedby heavy metal oxides selected from oxides of metals from the group:sodium, aluminium, potassium, calcium, gallium, germanium, arsenic,selenium, strontium, yttrium, antimony, indium, zinc, barium, lanthanum,tellurium and tin and 0-50 mol % total chloride.

In the case of a heavy metal oxybromide glass, the glass may be formedby heavy metal oxides selected from oxides of metals from the group:sodium, aluminium, potassium, calcium, gallium, germanium, arsenic,selenium, strontium, yttrium, antimony, indium, zinc, barium, lanthanum,tellurium and tin and 0-50 mol % total bromide.

In the case of polymers, the polymer may be PMMA or any poly-x compound,such as polyacrylate, polycarbonate, polystyrene, polypropylene orpolyester, with specific commercial examples being Cytop (trade mark)and Teflon (trade mark. Active dopant material such as erbium or otherrare earth elements can be incorporated as desired. Hybrid fibresincorporating glass and polymer may also be provided, for example silicain combination with PMMA.

While stainless steel grade 303 may be a suitable extruder die materialfor the extrusion temperatures and pressures associated with manyglasses, in some cases different materials may be more appropriate. Forexample, if a particular glass requires a higher extrusion temperatureand/or pressure, stainless steel grade 303 may not be able to withstandthe extrusion process. Other metals, such as tungsten, molybdenum,tantalum, niobium, titanium, or associated alloys, may be required toform an extruder die. Ceramic materials may also be considered forglasses with high melting temperatures, such as silicate glasses.

The structural requirements of the extruder die material for polymerextrusion are likely to be more relaxed. For example, a polymer canepreform similar to those described above could be extruded with analuminium, or even a plastic, extruder die.

Device Applications

Extruded microstructured optical fibres can possess a much wider rangeof geometries than conventionally fabricated microstructured fibre andbe easily made from a wide range of compound glasses. This makes themparticularly well suited to a number of applications and they can beused in a large range of devices, some of which are now outlined below.

(a) Highly non-linear fibre for switching applications: When the higherthird order refractive index constant n₂ typical of compound glassmaterials is combined with the high degree of mode confinementachievable with microstructured fibre, compound glass microstructuredfibres could exhibit up to 10000 times the non-linearity of conventionalsilica fibre. Extremely short fibre based non-linear devices could thusbe made for telecom power pulses. For example, the n₂ of SF57 glass is20 times larger than that of pure silica at 1550 nm, and so amicrostructured SF57 fibre will have an effective non-linearity γ thatis 20 times larger than its silica equivalent with the same effectivemode area, hence in a device, an order of magnitude lower power could beused. Note that such fibres could be used for devices based on selfaction (in which the properties of a laser beam get modified by thenon-linearity at high intensities), or within devices based on crossaction (in which the high intensity of one beam (pump beam) is used tomodify the properties of a second beam (probe beam)). Specific processesthat can be used in such switches include simple Kerr effect inducedSelf Phase Modulation (SPM), and Cross Phase Modulation (CPM). Withcertain materials at certain wavelengths it is also possible to envisageusing resonant non-linearities such as Two Photon Absorption (TPA) andwhich will again be enhanced in small core holey fibres.

FIG. 21 shows an example non-linear device used for spectral broadeningof pulses. For example, consider a compound glass microstructured fibre580 with a small core diameter of 2 microns, length 1 metre and n₂ ofabout 100 times that of silica (as for GLS glass). The propagation of aninitially transform limited Gaussian pulse of approx. 1.7 W peak powerin 1 m of fibre should result in a 10-fold spectral broadening, forexample from 1 to 10 nm pulse half width. Alternatively, one can expressthe above example in terms of a maximal phase shift at the pulse centrei.e. a 1.7 W Gaussian pulse will generate a peak non-linear phase shiftof 8.6 radians after propagation through 1 m of fibre. Note that both ofthe above calculations neglect the effect of fibre dispersion.Dispersion can play a significant role in the non-linear propagation ofa short optical pulse and can for example result in effects such assoliton generation. Compound glass fibres offer for example thepossibility of soliton formation at wavelengths not possible withconventional silica fibres.

A range of possibilities exist for using these fibres as the basis for avariety of non-linear optical switches. These include Kerr-gate basedswitches, Sagnac loop mirrors, non-linear amplifying loop mirrors or anyother form of silica fibre based non-linear switches (see reference [8],the contents of which is incorporated herein by reference).

One specific example is of a 2R data regenerator based on a short lengthof small-core microstructured optical fibre. Such a device based on asilica microstructured fibre with an effective core area of approx. 3μm² at 1550 nm is described in reference [11]. As described above, ashort pulse travelling in the highly non-linear fibre undergoes spectralbroadening. If a narrowband filter offset from the original centralwavelength of the pulse is inserted after the fibre, only spectralcomponents that are generated non-linearity are transmitted. In theimplementation described in reference [11], a dielectric filter is usedas the filtering element, its central wavelength was offset by 1.9 nmfrom the pulse, and just 3.3 m of fibre was required. It is possible toenvisage using other forms of filter for the offset narrowband filteringfunction including amongst others; a fibre Bragg grating, acousto-optictunable filter or Fabry Perot interferometer. In this way, a non-linearthresholder is formed, which passes through and equalises high intensitypulses, and suppresses low-intensity input pulses. Such a device can actas a data regenerator in a telecommunications system. By using a glasswith a higher n₂ such as SF57, SF59, tellurite or GLS glass, the figureof merit for this device would be even further improved relative tosilica. Note that for many applications of the above form of switch itis advantageous to use a fibre designed to have a normal group velocitydispersion at the operating wavelength since fibre with anomalousdispersion can in certain instances generate additional amplitude noisethrough soliton based effects. In other forms of switch however, mostspecifically those employing soliton effects for switching, anomalousdispersion is required.

(b) Raman Devices: The demand for optical data transmission capacity hasgenerated enormous interest in communication bands outside of aconventional erbium doped fibre amplifier (EDFA) gain bandwidth. Fibreamplifiers based on the Raman effect offer an attractive route towardsextending the range of accessible amplification bands. In addition toapplications in signal amplification, the fast response time (<10 fs) ofthe Raman effect can also be used for all-optical ultra-fast signalprocessing applications. One significant drawback to devices based onRaman effects in conventional optical fibres is that long lengths offibre (˜10 km) are generally required. To obtain adequate gain in ashort length of optical fibre it is necessary to use a speciality fibrewith either a very high Raman gain coefficient or a small effective modearea Hence microstructured fibres according to the invention are idealfor Raman amplification and modulation devices.

FIG. 22 schematically shows the operational implementation of a specific(pulsed) Raman amplifier by graphically representing the spectralcomponents in wavelength space. The pump source (P) was a 1536 nm diodeseeded, fibre amplifier based master oscillator power amplifier (MOPA)configuration, operated in pulsed mode to provide 20 ns square pulse at500 KHz repetition rate, corresponding to a 100:1 pump duty cycle. Pumpand input signal (I) beams are combined using a 1530/1630 nm wavelengthdivision multiplex coupler prior to launching the light into themicrostructured fibre Raman amplifier. A continuous wave external cavitytuneable laser was used to provide signal light (I) in the L+ wavelengthband (1600-1640 nm). In this particular implementation themicrostructured fibre was based on silica glass with a peak Raman shift(Δf) of ˜13 THz. The Raman gain peak (GP) was thus located at 1647 nmsuperimposed on the background amplified spontaneous emission signal.Higher gain and a lower noise figure are observed as the probe signalwavelength approaches the peak of the Raman gain curve (near 1650 nm).The Raman shifts in other glasses can be substantially different both interms of gain coefficient, and Raman lineshape. This opens up newpossibilities both for amplification bands (e.g. peaked at eitherlonger/shorter wavelength separations from the pump, and with differentlineshape relative to silica), and pump wavelengths for a givenamplification band, and promises far shorter device lengths/reduced pumppowers relative to silica based devices.

The Raman effect can also be used for signal modulation devices. In thisinstance, a strong pump beam is used to induce loss for a shorterwavelength co-propagating beam. In order to demonstrate this effect weused the same experimental configuration as used for Raman amplificationprocess schematically indicated in FIG. 22, except that the tuneablesignal source at around 1600 nm was now replaced with a 1458 nmcontinuous wave semiconductor diode laser. Strong pump pulses generate acorresponding signal loss due to stimulated Raman scattering (SRS),which results in the formation of ‘dark’ pulses at the signalwavelength, where the signal overlaps the pump pulses.

The Raman effect can also be used to make Raman laser devices (see forexample reference [13] for a specific embodiment of a microstructuredsilica fibre based Raman laser. To construct a Raman laser it isnecessary to take a Raman amplifier and to incorporate it within aresonant cavity, often defined as in reference [12] by using Fresnelfeedback from the fibre end facets themselves. The use of extrudedcompound glass microstructured fibres with different Raman gaincharacteristics should open up possibilities for Raman lasers at newwavelengths, with reduced thresholds (relative to other silica fibrebased Raman lasers), and new pump laser choices for specific Raman laseroperating wavelengths.

(c) Brillouin laser: Microstructured fibre according to the inventioncan also be applied to another important class of non-linear fibre-opticdevices—devices based on the Brillouin effect. This should includedevices based on stimulated Brillouin effects e.g. Brillouin laser andamplifier devices, and devices based on spontaneous Brillouin effects(e.g. distributed temperature/strain sensors).

FIG. 23 schematically represents an example Brillouin laser device 702.The pump source 700 for the microstructured fibre Brillouin laser isbased on an erbium fibre distributed feedback (DFB) seed laser 704coupled to a high power Er/Yb amplifier 706 by a fibre 708 containing anisolator 710. A Fabry-Perot resonator is formed by a 75 m length ofmicrostructured fibre 712, coupled by a lens 716 to a high-reflectivitycavity mirror 714 and by a 96% output coupler defined by the Fresnelreflection from the cleaved fibre facet at the pump launch end of thecavity. Power from the pump source 700 is coupled into the Fabry-Perotresonator via a lens 718. A beam splitter diverts a fraction of the pumpbeam to a pump monitor 722 and a fraction of the output beam to anoutput monitor 724. The frequency of the Brillouin laser output wasdownshifled (in this example by 10.6 GHz) relative to the pumpfrequency. The small core fibre provides good power conversionefficiency within the Brillouin laser device.

(d) Multicore fibre devices: Microstructured fibres according to theinvention may incorporate multiple cores as described above, and suchfibres can be used to make a range of practical devices. Some examplesinclude the switching of light between different cores of a multicorefibre, e.g. by detuning/tuning a particular coupling process via anon-linear effects, or through bending or deformation of the fibre asused in a variety of fibre sensing applications.

(e) Devices based on supercontinuum: When small core dimensions arecombined with the unusual dispersion properties possible in these novelmicrostructured fibre designs, it is possible to generate a broadsupercontinuum spectrum from a narrowband pulsed source by takingadvantage of non-linear processes in the fibre. New frequencies arecreated most efficiently when the fibre is pumped at or near the zerodispersion wavelength, and the generated supercontinuum can extend fromthe ultraviolet (UV) (<300 nm) out beyond 1.8 μm, and microstructuredfibres can be effectively single mode over this broad wavelength range.Applications of this phenomenon include: new source wavelengths, pulsecompression, metrology and spectroscopy. Compound glasses offer somespecific advantages for devices based on supercontinuum generation: (1)enhanced non-linearity (via enhanced n₂), resulting in supercontinuumgeneration at lower pulse energies (2) a wider range of zero dispersionwavelengths in these different materials should allow a wider range ofpump sources to be used (3) the enhanced transmission of some compoundglasses in the infrared (IR) opens the possibility extending thebroadband continuum into the IR.

(f) 1300 mm Optical Amplifier/laser: FIG. 24 shows a 1300 nm bandrare-earth doped microstructured fibre amplifier incorporatingmicrostructured optical fibre according to the invention. Pump radiationat 1020 nm from a laser diode and a 1300 nm input signal are supplied tofused coupler input arms 544 and 546, and mixed in a fused region 542 ofthe coupler. A portion of the mixed pump and signal light is supplied byan output arm 545 of the coupler to a section of Pr³⁺-doped galliumlanthanum sulphide microstructured fibre 540 where it is amplified andoutput. Other rare-earth dopants such as Nd or Dy could also be usedwith an appropriate choice of pump wavelength.

(g) Infrared Fibre amplifiers/laser: With compound glasses, a wide rangeof laser transitions become efficient and viable, so compound glassmicrostructured fibres according to the invention have potential for useas gain media in laser sources. Some examples include using lines at 3.6and 4.5 microns (Er), 5.1 microns (Nd³⁺), 3.4 microns (Pr³⁺), 4.3microns (Dy³⁺), etc. More examples for gallium lanthanum sulphide aregiven in reference [7] which is incorporated herein by reference. Thesetransitions could be exploited in a range of lasers, includingcontinuous wave, Q-switched, and mode-locked lasers and amplifiers. Inaddition, any of the usual rare-earth dopants could be considereddepending on the wavelengths desired.

FIG. 25 shows one example of an infrared fibre laser in the form of alaser having an erbium-doped gallium lanthanum sulphide microstructuredfibre gain medium 554 bounded by a cavity defined by a dichroic mirror552 and output coupler 556. Pump radiation at 980 nm from a laser diode(not shown) is supplied to the cavity through a suitable lens 550. Thelaser produces a 3.6 micron laser output. It will be appreciated thatother forms of cavity mirrors could be used, e.g. in-fibre Bragg gratingreflectors. The fibre laser cavity could also be configured in atravelling wave ring geometry.

(h) High-Power Cladding Pumped Lasers and amplifiers: The higher indexcontrast possible in compound glass microstructured fibres allows forfibres with very high numerical aperture (NA) of well in excess ofunity. It is therefore possible to provide improved pump confinement andthus tighter focusing, shorter devices, lower thresholds etc.

FIG. 26 shows one example in the form of a cladding pumped laser havinga lead glass microstructured fibre such as SF57 gain medium 566 dopedwith Nd. A pump source is provided in the form of a high-powerbroad-stripe diode 560 of 10 W total output power at 815 nm. The pumpsource is coupled into the gain medium through a focusing lens 562 andthe cavity is formed by a dichroic mirror 564 and output coupler 568 toprovide high-power, multiwatt laser output at 1.08 microns.

(i) Evanescent Field Devices: The guided mode can be made to havesignificant overlap with gas or liquid present in the holes, so thatfibres can be used to measure gas concentrations, for example. Aparticular advantage of compound glass microstructured fibres is thatlonger wavelengths can be used, which would allow a much wider range ofgases to be detected. The mid-infrared (3-5 microns) part of thespectrum is of particular interest.

Working at these longer wavelengths should also significantly ease thefabrication requirements associated with making microstructured fibresthat are suitable for evanescent field devices, simply because the sizeof the structure that is required scales with the wavelength.

FIG. 27 shows a transverse section of an example glass microstructuredfibre according to the invention for gas sensing. Large holes 586 in thecladding are provided by radially extending strut structures extendingbetween a solid core 584 and outer wall 582. The core diameter ‘d’ ispreferably much less than the operating wavelength ‘λ’ to ensure that asignificant fraction of the mode power lies in the microstructuredregion. For example, for 5 micron operation a core diameter of 2 micronscould be used.

FIG. 28 shows a sensing device including a gallium lanthanum sulphidemicrostructured fibre 592 having a structure as shown in FIG. 25. Thegallium lanthanum sulphide microstructured fibre 592 is arranged in agas container 590, containing CO₂ gas, for example. A light source 598is arranged to couple light into the gallium lanthanum sulphidemicrostructured fibre 592 via a coupling lens 594 through a window inthe gas container. Light is coupled out of the gas container through afurther lens 596 and to a detector 599. The detector will registerpresence of a particular gas through an absorption measurement of thelight (for example, absorption of light at 4.2 microns for the detectionof CO₂). Tellurite glasses also offer transmission further into theinfrared than silica fibres, and so similar devices based on telluriteglasses could be envisaged.

(j) Non-linear grating based devices: The high non-linearity fibremanufacturable with the invention should allow for low threshold gratingbased devices (logic gates, pulse compressor and generators, switchesetc.). For example, FIG. 29 shows an optical switch based on galliumlanthanum sulphide microstructured fibre 600 made with a small corediameter of around 1-2 microns and incorporating an optically writtengrating 602. In operation, pulses at low power (solid lines in thefigure) are reflected from the grating, whereas higher power pulses(dashed lines in the figure) are transmitted due to detuning of thegrating band gap through Kerr non-linearity.

(k) Acoustic Devices: More efficient microstructured fibre acousto-optic(AO) devices can be fabricated. The acoustic figure of merit in compoundglasses is expected to be as much as 100-1000 times that of silica Thisopens the possibility of more efficient fibre AO devices such asAO-frequency shifters, switches etc. Passive stabilisation of pulsedlasers may also be provided. Microstructured fibres might also allowresonant enhancements for AO devices via matching of the scale ofstructural features to a fundamental/harmonic of the relevant acousticmodes. The use of compound glass materials would also allow AO devicesto be extended to the infrared.

FIG. 30 shows an AO device in the form of a null coupler based ongallium lanthanum sulphide microstructured fibre. The device has theform of a null coupler 614 with a coupling region at which apiezoelectric transducer 610 is arranged for generating acoustic waves.In the absence of an acoustic wave, light I is coupled from a source 612into one output arm of the coupler (solid line), whereas in the presenceof the acoustic wave light is coupled into the other output of thecoupler (dashed line). Further details of devices of this kind can befound in references [9] and [10].

(l) Highly non-linear fibre for second harmonic generation (SHG): Thehigher third order refractive index constant n₂ typical of compoundglass materials can be combined with the high degree of mode confinementachievable with microstructured fibres according to the invention toprovide up to 10000 times the effective non-linearity of conventionalsilica fibre. Efficient short fibre based non-linear devices could thusbe made based on third order effects. In materials, such as glass andmany polymers, inversion symmetry at the molecular level means that thematerial and indeed any fibre made of such materials cannot possess asecond order non-linearity. However, within certain materials, mostnotably certain polymers, and glasses, it is possible to use polingtechniques to induce a large, permanent, “frozen in” DC electric fieldwithin the material. This internal DC electric field in combination withthe third order non-linearity can then give rise to large values ofeffective second order non-linearity. It is possible to pole thematerial within the core of an optical fibre. Moreover, it is possibleto create periodically poled sections of fibre along the fibres lengthso as to create a second order non-linearity grating. The pitch of thisgrating can be tailored so to phase-match a specific non-linear processbetween three optical fields propagating within the fibre. This form ofphase matching employing periodically poled regions of non-linearity isgenerally referred to as quasi-phase matching. Specific non-linearprocesses that can be phase matched include second harmonic generation,and both sum and frequency difference generation.

FIG. 31 shows a schematic longitudinal axial section through amicrostructured optical fibre 620 fabricated from a preform extrudedfrom the die shown in FIG. 20 xiv for use in a forward-interactionsecond harmonic generator (SHG) device. The periodically poledsecond-order non-linearity in the core 622 is shown schematically byblack and white striping in the figure. The poling electrodes 624, 625are formed within the drawn hollow cores of the cane preform. The drawnouter wall 626 of the preform is also shown.

(m) Highly non-linear fibre for three wave mixing (TWM): FIG. 32 shows abackward TWM fibre device that provides a transparent and effectivefrequency converter, which would be largely employed inWavelength-Division-Multiplexing (WDM) optical telecommunicationsystems. The pump beam interacting with the non-linear microstructuredfibre and with the incoming signal, produces a backward travelling idlerwhich carries the same modulation as the signal at a differentwavelength such that: ω_(i)+ω_(s)=ω_(p) where ω_(i), ω_(s), ω_(p) areused to denote idler, signal and pump frequency respectively. Thephase-matching condition is provided by the use of a periodicnon-linearity achieved in the core by conventional thermal poling, it isnoted that the period a required for the poling is much smaller than forforward-interaction devices, typically of the order of a micron or less,so that use of a phase mask, rather than an amplitude mask, may bepreferred for the poling. The small poling period is needed in order tocompensate for the large momentum mismatch between thecounter-propagating waves.

An advantage of backward interaction is the separation between thesignal and the idler and pump, which occurs naturally. A wavelengthconverter based on such a device would not therefore require any furtheroptical filtering to separate the desired wavelength (idler) from theresidual ones (pump and signal).

Another application of backward-interaction TWM is for theimplementation of mirror-less optical parametric oscillators, where theoptical feedback required in order to start the oscillation is providedby the backward propagation of the waves inside the non-linear fibre.

(n) Highly non-linear fibre for Four-Wave-Mixing WM) processes: Thehigher third order refractive index constant n₂ typical of compoundglass materials can be combined with the high degree of mode confinementachievable with microstructured fibres according to the invention toprovide up to 10000 times the effective non-linearity of conventionalsilica fibre. Efficient short fibre based non-linear devices should thusbe possible based on 4-wave mixing. In order to achieve efficient 4-wavemixing processes in fibre one need to ensure both (a) energyconservation, and (b) phase matching (momentum conservation), for thephotons involved in the specific desired process. Phase matching can beachieved in a variety of ways within a fibre for example between fourphotons in a single fundamental polarisation mode of the fibre, betweenphotons in different polarisation/spatial modes, between photons in thefundamental and higher order transverse modes, and between photonsexclusively in higher order transverse modes of the fibre. The linearproperties of the waveguide e.g. group velocity, group velocitydispersion, birefringence and modal overlap of the fundamental andhigher-order modes of the structure thus play a critical role indefining which specific non-linear processes can be efficient in a givenfibre. Each of these properties can be tailored to a greater extent inmicrostructured fibres than in conventional fibres allowing for anincreased range of phase-matching possibilities, and therefore anincreased range of efficient non-linear four wave mixing processes.Obviously the higher non-linear coefficient of materials such ascompound glass can greatly reduce the powers required to make a givenphase-matched process efficient. Specific four wave mixing processesinvolving the generation of photons at different frequencies include:Third Harmonic Generation (THG), degenerate 4-wave mixing (parametricamplification and lasing), non-degenerate four wave mixing, andmodulational instability. Such processes can be used as the basis of avariety optical devices, including amongst others devices for wavelengthconversion, optical switching, amplification (and lasing),demultiplexing, phase conjugation and dispersion compensation of anincoming laser beam/signal.

Many other devices can incorporate microstructured optical fibreaccording to the invention. The above examples are merely illustrative.

REFERENCES

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1. An extruder die for forming a preform for manufacture into an opticalfiber, comprising: a central feed channel for receiving a materialsupply by pressure-induced fluid flow; flow diversion channels arrangedto divert a first component of the material radially outwards into awelding chamber formed within the die; a core forming conduit arrangedto receive a second component of the material from the central feedchannel that has continued its onward flow; and a nozzle having an outerpart in flow communication with the welding chamber and an inner part inflow communication with the core forming conduit, to respectively definean outer wall and core of the preform.
 2. An extruder die according toclaim 1, wherein the die is provided with pairs of mutually facinginternal walls that form gaps extending between the core forming conduitand the welding chamber and allow fluid communication therebetween, thegaps being shaped to form struts supporting the core in the outer wall.3. An extruder die according to claim 2, wherein the mutually facinginternal walls incorporate at least one bend in order to increase theradial length of the struts.
 4. An extruder die according to claim 2,wherein the internal walls have a radial length greater than the gapwidth.
 5. An extruder die according to claim 4, wherein the radiallength of the internal walls is greater than the gap width by a factorof one of: 2, 3, 4, 5, 6, 7, 8, 9, 10 and
 20. 6. An extruder dieaccording to claim 1, wherein the outer part of the nozzle is shaped toprovide a circular-section preform outer wall.
 7. An extruder dieaccording to claim 1, wherein the outer part of the nozzle deviates froma circular shape so as to provide sections of preform wallinterconnecting wall-to-strut junctions that are shorter than would berequired to form a circular-section preform outer wall.
 8. An extruderdie according to claim 1, wherein the outer part of the nozzle has afirst dimension defining a wall thickness of the preform outer wall andwherein said first dimension is greater than said gap between themutually facing internal walls that form the preform struts.
 9. Anextruder die according to claim 8, wherein said first dimension isgreater than said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and10.
 10. An extruder die according claim 1, wherein the inner part of thenozzle has a second dimension defining a core thickness of the preformcore and wherein said second dimension is greater than said gap betweenthe mutually facing internal walls that form the preform struts.
 11. Anextruder die according to claim 10, wherein said second dimension isgreater than said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and10.
 12. An extruder die according to claim 1, wherein the flow diversionchannels include a first group of the flow diversion channels whichextend from the core forming conduit to the welding chamber.
 13. Anextruder die according to claim 12, wherein the flow diversion channelsof the first group extend perpendicular to the core forming conduit. 14.An extruder die according to claim 12, wherein the flow diversionchannels of the first group have a width dimension that is substantiallyconstant in the feed direction.
 15. An extruder die according to claim12, wherein the flow diversion channels of the first group have a widthdimension that reduces in the feed direction.
 16. An extruder dieaccording to claim 1, wherein the flow diversion channels include asecond group of the flow diversion channels that extend from the centralfeed channel to the welding chamber.
 17. An extruder die according toclaim 16, wherein the flow diversion channels of the second group extendobliquely to the central feed channel.
 18. An extruder die according toclaim 1, further comprising a mandrel extending down the central feedchannel into the core forming conduit with a dependent peg thereof so asto form a hollow core in the preform.
 19. An extruder apparatusincluding a main body having a location for receiving an extruder dieaccording to claim 1, a space for arranging a billet of material abovethe extruder die and a force transmitting assembly for applying pressureto the billet to drive the material through the extruder die.
 20. Amethod of forming a preform for manufacture into an optical fiber,comprising: applying pressure to supply a material into a central feedchannel of an extruder die by pressure-induced fluid flow; diverting afirst component of the material radially outwards into a welding chamberformed within the die; allowing a second component of the material toflow onwards from the central feed channel into a core forming conduitin the die; and dispensing the material through a nozzle having an outerpart in flow communication with the welding chamber and an inner part inflow communication with the core forming conduit, to respectively definean outer wall and core of the preform.
 21. A method according to claim20, wherein the extruder die is provided with pairs of mutually facinginternal walls that form gaps extending between the core forming conduitand the welding chamber and allow fluid communication therebetween, thegaps being shaped to form struts supporting the core in the outer wall.22. A method according to claim 21, wherein the mutually facing internalwalls incorporate at least one bend in order to increase the radiallength of the struts.
 23. A method according to claim 20, wherein theinternal walls have a radial length greater than the gap width.
 24. Amethod according to claim 23, wherein the radial length of the internalwalls is greater than the gap width by a factor of one of: 2, 3, 4, 5,6, 7, 8, 9, 10 and
 20. 25. A method according to claim 20, wherein theouter part of the nozzle is shaped to provide a circular-section preformouter wall.
 26. A method according to claim 20, wherein the outer partof the nozzle deviates from a circular shape so as to provide sectionsof preform wall interconnecting wall-to-strut junctions that are shorterthan would be required to form a circular-section preform outer wall.27. A method according to claim 20, wherein the outer part of the nozzlehas a first dimension defining a wall thickness of the preform outerwall and wherein said first dimension is greater than said gap betweenthe mutually facing internal walls that form the preform struts.
 28. Amethod according to claim 27, wherein said first dimension is greaterthan said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and
 10. 29.A method according to claim 20, wherein the inner part of the nozzle hasa second dimension defining a core thickness of the preform core andwherein said second dimension is greater than said gap between themutually facing internal walls that form the preform struts.
 30. Amethod according to claim 29, wherein said second dimension is greaterthan said gap by a factor of one of: 2, 3, 4, 5, 6, 7, 8, 9 and
 10. 31.A method according to claim 20, wherein the flow diversion channelsinclude a first group of the flow diversion channels which extend fromthe core forming conduit to the welding chamber.
 32. A method accordingto claim 31, wherein the flow diversion channels of the first groupextend perpendicular to the core forming conduit.
 33. A method accordingto claim 31, wherein the flow diversion channels of the first group havea width dimension that is substantially constant in the feed direction.34. A method according to claim 31, wherein the flow diversion channelsof the first group have a width dimension that tapers down in the feeddirection.
 35. A method according to claim 20, wherein the flowdiversion channels include a second group of the flow diversion channelswhich extend from the central feed channel to the welding chamber.
 36. Amethod according to claim 35, wherein the flow diversion channels of thesecond group extend obliquely to the central feed channel.
 37. A methodaccording to claim 20, wherein the extruder die further comprises amandrel extending down the central feed channel into the core formingconduit with a dependent peg thereof so as to form a hollow core in thepreform.
 38. A method according to claim 20, wherein the materialsupplied to the central feed channel is a glass.
 39. A method accordingto claim 20, wherein the material supplied to the central feed channelis a polymer.
 40. A method of manufacturing an optical fiber comprising:forming a preform by extrusion according to the method of claim 20; andreducing the preform to an optical fiber.
 41. A method according toclaim 40, wherein reducing the preform to an optical fiber comprisesreducing the preform to a cane followed by reducing the cane to theoptical fiber.
 42. A method according to claim 41, wherein reducing thecane comprises arranging the cane in a tubular jacket and reducing thecane and tubular jacket into the optical fiber.
 43. A method accordingto claim 41, wherein reducing the cane comprises arranging the caneamongst a plurality of rods and/or tubes to form a stack and reducingthe stack into the optical fiber.
 44. A preform for manufacture into anoptical fiber made using the method of claim
 20. 45. An optical fibermade using the method of claim
 40. 46. A preform for manufacture into anoptical fiber, comprising a core suspended in an outer wall by aplurality of struts.
 47. A preform according to claim 46, wherein thestruts have a width dimension smaller than a width dimension of at leastone of the outer wall and the core by a factor of at least two.
 48. Apreform according to claim 47, wherein the factor is at least one of3,4,5,6,7,8,9 and
 10. 49. A preform according to claim 46, wherein thestruts incorporate at least one bend in order to increase their radiallength.
 50. A preform according to claim 46, wherein the wall as viewedin cross-section deviates from a circular shape so as to provide wallsections interconnecting wall-to-strut junctions that are shorter thanwould be required to form a circular-section outer wall.
 51. A preformaccording to claim 46, wherein the core has a thickness that variesalong its axial extent.
 52. A preform according to claim 46, wherein thestruts extend helically.
 53. A preform according to claim 46 includingat least one further core.
 54. A preform according to claim 46 includingat least one integral electrode.
 55. A preform according to claim 46,wherein the struts have a width and a radial length and the radiallength is greater than the width.
 56. A preform according to claim 55,wherein the radial length of the struts is greater than the width by afactor of one of: 2, 3, 4, 5, 6, 7, 8, 9, 10 and
 20. 57. A preformaccording to claim 46, made of a glass material.
 58. A preform accordingto claim 46, made of a polymer material.
 59. A preform according toclaim 46, wherein the core is hollow.
 60. An optical fiber comprising acore suspended in an outer wall by a plurality of struts.
 61. An opticalfiber according to claim 60, wherein the struts have a width dimensionsmaller than a width dimension of at least one of the outer wall and thecore by a factor of at least two.
 62. An optical fiber according toclaim 61, wherein the factor is at least one of 3, 4, 5, 6, 7, 8, 9 and10.
 63. An optical fiber according to claim 60, wherein the core has athickness that varies along its axial extent.
 64. An optical fiberaccording to claim 60 including at least one further core.
 65. Anoptical fiber preform according to claim 60, wherein the struts extendhelically.
 66. An optical fiber according to claim 60 including at leastone integral electrode.
 67. An optical fiber according to claim 60,wherein the struts have a radial length greater than at least one of 2,3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18 and 20 micrometers.
 68. Anoptical fiber according to claim 67, wherein the struts have a widthsmaller than the radial length of the struts by a factor of at least oneof 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18 and
 20. 69. An opticalfiber according to claim 60, made of a glass material.
 70. An opticalfiber according to claim 60, made of a polymer material.
 71. An opticalfiber according to claim 60, having a core width of greater than atleast one of: 0.3, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18and 20 micrometers.
 72. An optical fiber according to claim 60, whereinthe core is hollow.
 73. A method of manufacturing a microstructuredoptical fiber comprising: forming by extrusion a preform comprising acore suspended in an outer wall by a plurality of struts; and reducingthe preform into an optical fiber.
 74. A laser, amplifier, non-lineardevice, switch, acousto-optic, sensor or other optical device comprisingoptical fiber according to claim 60.